Berkeleyan

Research Roundup

02 February 2005

Missing link found between the whale and its closest relative, the hippo

A group of four-footed mammals that flourished worldwide for 40 million years and then died out in the ice ages is the missing link between the whale and its not-so-obvious nearest relative, the hippopotamus.

The conclusion by Berkeley postdoctoral fellow Jean-Renaud Boisserie and his French colleagues finally puts to rest the longstanding notion that the hippo is actually related to the pig or to its close relative, the South American peccary. The finding thus reconciles the fossil record with the 20-year-old claim that molecular evidence points to the whale as the closest relative of the hippo.

“The problem with hippos is, if you look at the general shape of the animal, it could be related to horses, as the ancient Greeks thought, or pigs, as modern scientists thought, while molecular phylogeny shows a close relationship with whales,” said Boisserie. “But cetaceans — whales, porpoises and dolphins — don’t look anything like hippos. There is a 40-million-year gap between fossils of early cetaceans and early hippos.”

In a paper appearing in the Online Early Edition of the Proceedings of the National Academy of Sciences, Boisserie and colleagues Michel Brunet and Fabrice Lihoreau fill in this gap by proposing that whales and hippos had a common water-loving ancestor 50 to 60 million years ago that evolved and split into two groups: the early cetaceans, which eventually spurned land altogether and became totally aquatic; and a large and diverse group of four-legged beasts called anthracotheres. The pig-like anthracotheres, which blossomed over a 40-million-year period into at least 37 distinct genera on all continents except Oceania and South America, died out less than 2.5 million years ago, leaving only one descendant: the hippopotamus.

This proposal places whales squarely within the large group of cloven-hoofed mammals (even-toed ungulates) known collectively as the Artiodactyla — the group that includes cows, pigs, sheep, antelopes, camels, giraffes, and most of the large land animals. Rather than separating whales from the rest of the mammals, the new study supports a 1997 proposal to place the legless whales and dolphins together with the cloven-hoofed mammals in a group named Cetartiodactyla.

Though most biologists now agree that whales and hippos are first cousins, they continue to clash over how whales and hippos are related, and where they belong within the even-toed ungulates, the artiodactyls. A major roadblock to linking whales with hippos was the lack of any fossils that appeared intermediate between the two.

This new analysis finally brings the fossil evidence into accord with the molecular data, showing that whales and hippos indeed are one another’s closest relatives.

— Robert Sanders

Deep tremors under San Andreas Fault could portend earthquakes

Berkeley seismologists have discovered mysterious tremors deep under the San Andreas Fault that may portend future earthquakes.

The continuous tremors are “a kind of chatter” emanating from a depth of 20 to 40 kilometers below the surface, near the boundary between the Earth’s crust and the hot mantle and much deeper than the 15-kilometer limit of most earthquakes, said study leader Robert Nadeau, an assistant research seismologist at the Berkeley Seismological Laboratory. Most of the tremors are five times deeper than the average quake on this segment of the fault.

The faint tremors, which were detected beneath the town of Cholame, 15 miles southeast of Parkfield, are similar to those discovered in the past two years at subduction zones in Japan and the Pacific Northwest. This is the first time, however, that such tremors have been recorded under a transform fault. At a subduction zone, one of the Earth’s plates slides under another at roughly a 45-degree angle. Transform faults, on the other hand, slide horizontally against one another. The most common type of fault in California slides this way, where the slipping surface is nearly vertical.

Variations in the low-amplitude, low-frequency tremors, which last more than four minutes each, occur several weeks before variations in the rate of microquakes on the San Andreas Fault, Nadeau said, suggesting that deformation associated with the tremors may cause the small quakes. The tremors have gone undetected until now because earthquake monitoring instruments to date have ignored continuous activity and only recorded jerky, episodic shaking. Now that it’s possible to store and analyze large amounts of information, seismologists like Nadeau can look at long records of activity in search of patterns.

“This is new information from an area deep down under the fault we have not been able to look at before,” Nadeau said. “If these tremors are precursory to earthquakes, there is potential here for earthquake forecasting and prediction.”

A new study led by researchers at Berkeley, the National Cancer Institute, and the Chinese Center for Disease Control and Prevention has found that people exposed to low levels of the chemical benzene in the workplace had significantly lower blood-cell counts than did workers who were not exposed.

The researchers found that white-blood-cell and platelet counts were lower even with exposure levels below one part benzene per million parts air, or 1 ppm. They also found that benzene exposure significantly lowered the number of progenitor cells, which include stem cells, in the blood. These stem and progenitor cells are precursors to all blood cells. The findings on progenitor cells strengthen the link between benzene and leukemia, a cancer that begins with mutations in blood stem cells, according to the researchers.

The study, published recently in the journal Science and sponsored by the National Institutes of Health, was conducted in Tianjin, China, from 2000 to 2001. Researchers compared blood and urine samples from 250 people exposed to low levels of benzene in shoe-manufacturing factories with a control group of 140 people working in clothing factories who were not exposed to benzene. They also monitored the levels of benzene in the air over the course of 12 to 16 months.

“Most prior studies focused on exposures to higher levels of benzene at work,” said Luoping Zhang, a researcher at Berkeley’s School of Public Health and co-lead author of the study. “Our study found that benzene had an impact on blood cell counts at lower levels of exposure.”

The study’s senior author, Martyn Smith, a Berkeley professor of toxicology, added: “We need more studies to fully understand what these changes mean. We need to look into what other kinds of biologic changes may be happening after benzene exposure in the bone marrow where blood cells are formed.”